Hybrid plants result from crosses between two different varieties or inbreds. Hybrids are often superior over non-hybrid varieties in vigor, yield, uniformity, as well as in other characters. And this is the main reason for their agricultural value. In producing hybrid seed, the two varieties are grown in proximity for effective pollination; one variety serves as seed parent and the other as pollen parent. Natural pollination is brought about by either wind, insects, or other animals. In the past, crossing two varieties on a large scale was a very difficult task because the plants of many crop species bear both male and female reproductive structures, i.e., they are capable of self-pollination as well as cross-pollination. Therefore, in order to prevent self-pollination and obtain pure hybrid seed, it was necessary to emasculate the seed parent by hand, a costly operation which usually requires a large number of workers. Emasculation in this case refers to a physical removal of functional male reproductive structures from the seed parent.
Different systems were devised which do away with the problem of emasculation and thus facilitate the commercial production of hybrid seed. Some effective systems are based on the use of male-sterile plants, or self-incompatible plants, or female plants as seed parents. An important feature of these systems is that they enable their male-sterile, self-incompatible, and female plants to reproduce their kind in practical ways.
These systems are as follows:
1. A group of genetic elements which controls cytoplasmic male-sterility in the seed parent, the reproduction of this seed parent by a "maintainer" line, and the restoration of male fertility in the hybrid plants. This system proved to be very successful in hybrid seed production in a number of economically important crops such as maize (corn), sorghum, and sunflower. It is described in Breeding Field Crops by J. M. Poehlman, Avis Publishing Company, Inc., Westport, Conn. (1977), and in other plant breeding texts.
2. Combinations of system #1 and genetic elements Which either improve cross-pollination or increase the usefulness of the hybrids. Such combinations are described in U.S. Pat. Nos. 4,351,130 (1982), 4,378,655 (1983), and 4,517,763 (1985) for wheat, sunflower, and rape, respectively.
3. Clonal propagation of genetic self-incompatible plants. This is described in U.S. Pat. No. 4,499,687 (1985) for use in hybrid Brassica seed production.
4. Mutants which appear as female plants in one environment and as monoecious plants--each bearing both female and male flowers--in another environment. See Shifriss, O. (1960). "Conventional and Unconventional Systems Controlling Sex Variations in Ricinus". J. Genet. 57: 361-388.
5. Chemical agents which convert monoecious (or hermaphroditic) plants into female (or male-sterile) plants. Example: potentially monoecious plants of squash (Cucurbita pepo L.) can be converted into female plants by a treatment with 2-chloroethylphosphonic acid (sold under several designations including Ethephon).
6. A combination of genetic elements and chemical agents which control sex expression in plants. Example: in cucumbers (Cucumis sativus L.), genetic female plants can be converted into monoecious plants by treatment with either gibberellic acid (GA.sub.3 or other gibberellins) or silver nitrate.
The literature on systems #5 and #6 is reviewed in Pollination Mechanisms, Reproduction and Plant Breeding by R. Frankel and E. Galun, Springer--Verlag, Berlin Heidelberg New York (1977). These systems are being used in commercial hybrid seed production of several members of the Cucurbitaceae family.
The present invention relates to a synthesis of genetic females as well as to the utilization of these females in squash. The name "squash" refers here to any variety of the 5 cultivated species of the botanical genus Cucurbita: C. ficifolia Bouche, C. maxima Duch., C. mixta Pang., C. moschata Poir., and C. pepo L. The illustrative species in which the new genetic females were synthesized is C. pepo.
The 5 cultivated squash species are monoecious (FIG. 1; see also Nitsch, J. P., E. B. Kurtz, Jr., J. L. Liverman, and F. W. Went. 1952. "The Development of Sex Expression in Cucurbit Flowers". Amer. J. Bot. 39:32-43). Although some of these species are restricted to certain ecological regions, squash has become a cosmopolitan crop. It is now grown extensively in many parts of the world, particularly by small gardeners, and its popularity as food crop is increasing.
No useful cytoplasmic male-sterile plants, or self-incompatible plants, or genetic female plants were reported in any variety of the cultivated cucurbita species. Whereas the genetic females of monoecious species of other plant genera were found to occur naturally, the new genetic females of squash were synthesized through crosses of monoecious inbreds followed by selection. But like some of the naturally occurring genetic females, the newly synthesized females can be converted into monoecious plants by chemical agents. Therefore, the new genetic females of squash can be classified under system #6.
The current commercial production of hybrid squash seed is based largely on system #5. This involves the use of Ethephon treatment for conversion of a potentially monoecious variety into a female population. And this chemically induced female population is used as seed parent in hybrid production. Bees are the main agent for cross-pollination. But the Ethephon treatment has the following shortcomings:
(1) Cost of Ethephon treatment. Two to six sprayings are often required for effective sex conversion. The number of sprayings depends on concentration, variety, and environmental conditions.
(2) Field inspections are necessary in order to determine the efficacy of the sprayings.
(3) Partial pruning of vines may be needed in order to eliminate the formation of male flowers some time after the last spraying.
(4) Ethephon treatment may slow down growth or stunt plants.
The use of the newly synthesized genetic females can obviate most of the above shortcomings.
The present invention is unique, economically valuable, and applicable to other monoecious species of plants. It is unique because it represents the first case in higher plants in which genetic females were synthesized through crosses of monoecious inbreds; it is economically valuable because it offers a more efficient method of hybrid seed production in squash than the one currently in use; and it is applicable to other monoecious species of plants because the genetic control of sex in some of these species is similar.